Missing link in metal physics explains Earth's magnetic field

January 28, 2015

Conception of Earth's core overlaid by the electronic structure of iron; the width (fuzziness) of the lines results from the electron-electron scattering. Image courtesy of Ronald Cohen. Credit: Ronald Cohen

Earth's magnetic field is crucial for our existence, as it shields the life on our planet's surface from deadly cosmic rays. It is generated by turbulent motions of liquid iron in Earth's core. Iron is a metal, which means it can easily conduct a flow of electrons that makes up an electric current. New findings from a team including Carnegie's Ronald Cohen and Peng Zhang shows that a missing piece of the traditional theory explaining why metals become less conductive when they are heated was needed to complete the puzzle that explains this field-generating process. Their work is published in Nature.

The center of the Earth is very hot, and the flow of heat from the planet's center towards the surface is thought to drive most of the dynamics of the Earth, ranging from volcanoes to plate tectonics. It has long been thought that heat flow drives what is called thermal convection—the hottest liquid becomes less dense and rises, as the cooler, more-dense liquid sinks—in Earth's liquid iron core and generates Earth's magnetic field. But recent calculations called this theory into question, launching new quests for its explanation.

In their work, Cohen and Zhang, along with Kristjan Haule of Rutgers University, used a new computational physics method and found that the original thermal convection theory was right all along. Their conclusion hinges on discovering that the classic theory of metals developed in the 1930's was incomplete.

The electrons in metals, such as the iron in Earth's core, carry current and heat. A material's resistivity impedes this flow. The classic theory of metals explains that resistivity increases with temperature, due to atoms vibrating more as the heat rises. The theory says that at high temperatures resistivity happens when electrons in the current bounce off of vibrating atoms. These bounced electrons scatter and resist the current flow. As temperature increases, the atoms vibrate more, and increasing the scattering of bounced electrons. The electrons not only carry charge, but also carry energy, so that thermal conductivity is proportional to the electrical conductivity.

The work that had purportedly thrown the decades-old prevailing theory on the generation of Earth's magnetic field out the window claimed that thermal convection could not drive magnetic-field generation. The calculations in those studies said that the resistivity of the molten metal in Earth's core, which is generated by this electron scattering process, would be too low, and thus the thermal conductivity too high, to allow thermal convection to generate the magnetic field.

Cohen, Zhang, and Haule's new work shows that the cause of about half of the resistivity generated was long neglected: it arises from electrons scattering off of each other, rather than off of atomic vibrations.

"We uncovered an effect that had been hiding in plain sight for 80 years," Cohen said. "And now the original dynamo theory works after all!"

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14 comments

"We uncovered an effect that had been hiding in plain sight for 80 years," Cohen said. "And now the original dynamo theory works after all!"

LMAO...sure...until the field reverses polarity without the planet shaking apart. C'mon guys...you realize that pinning this as the mechanism requires the rest of the physics to be adhered to...right?

There is one way to reverse polarity in a magnetic field caused by directional flow. If this is the mechanism producing the earths EM field, then based on observations of late it was interesting knowing you all.

Never let a little thing like explaining how the Earth's magnetic field can reverse polarity if it is produced by directional flow stop you from publishing and getting credit for another scientific paper. The peers should have caught this and asked the scientists to elaborate on how it is possible, but then again, maybe it would be better if the explanation is provided within a separate published paper, getting more mileage out of the computational effort. After all, since this was based on computation, not direct observation, just tweak the maths some more and you'll have it!

@ reset, exactly what does field reversal have to do with this finding? Nothing they said goes against current theory, as far as I can tell. Do you have a specific critique?

A magnetic field forms perpendicular to the current flow which creates it, this is the mechanism proposed to be at work here to produce the earths dipole field. In order for the field to reverse polarity, Billions of tons of liquid metal would have to slow, stop, and flow in the opposite direction. The seismic ramifications of this kind of kinetic transfer would fracture the entire crust. So if the field flips polarity and we have no earth quakes, it isn't generated the way the article claims to have been verified that it is by that last sentence.

"And now the original dynamo theory works after all!"

TO say nothing of a mechanism in the system that could cause such a reversal and not effect anything else but the earths EM field.

Folks, *please* do a little more research before spouting old notions swept away by 'Plate Tectonics', 'Continental Drift' etc etc, not to mention studying the oft-weird magnetic fields of other planets...

It is NOT one single bipole field down there in 'Middle Earth', but a slowly, slowly roiling zoo of competing field components, some of which actually point the opposite way to the current 'majority' field.

Hence 'magnetic variation', which every 'compass' navigator must master and track. Hence magnetic poles offset and drifting from rotation axis. Hence the odd zone known as the 'South Atlantic Anomaly' which bugs spacecraft due to the radiation it lets through...

IIRC, each 'reversal' goes via a tangled 'multipole' configuration as the balance of power ebbs & flows. IIRC, there's even evidence in the geology of some 'reversals' stalling, then reverting to prior direction...

@reset So you are implying that the reversal of the sun's magnetic field during every solar cycle is the sun's internal matter reversing direction? Where in the world would all of that energy for slowing, stopping, then reversing and speeding up that mass come from?

@reset So you are implying that the reversal of the sun's magnetic field during every solar cycle is the sun's internal matter reversing direction? Where in the world would all of that energy for slowing, stopping, then reversing and speeding up that mass come from?

I would imagine that thermal convection is generally constant while electronic convection varies substantially. The polarity of the field changes with the electric fluctuations, not the physical direction of material flow. Electronic flow could be influenced by extra solar phenomena.

@reset So you are implying that the reversal of the sun's magnetic field during every solar cycle is the sun's internal matter reversing direction? Where in the world would all of that energy for slowing, stopping, then reversing and speeding up that mass come from?

UM...I just said the same thing about earth...so...WoW...no. I am saying the magnetic field cannot be produced by a dynamo because of the variability of the field.

Nik...the earths magnetic field is a structure, definitely not a bipole field through the center of the planet. The rest of what you said may be useful information to some....but when a compass which points north at most locations flips to point south while in those same locations, you have a polarity flip...and now we are back to how, if it is a dynamo generating the field, this can occur. Can you address that specifically?

" It has long been thought that heat flow drives what is called thermal convection—the hottest liquid becomes less dense and rises, as the cooler, more-dense liquid sinks—in Earth's liquid iron core and generates Earth's magnetic field."

That's pretty much basic physics when it comes to molten metal but there is also another physical input that should be having some sort of effect here and that is the Earth is spinning and that should act somewhat like a centrifuge on the molten material in the core.

Technically that should have an opposite effect from what would normally happen in a pool of molten metal setting still. That has to have some effect on how the molten metal is circulating down there. I just don't have any real idea of how much in the overall picture.

Reset, the simplest analogy I can think of is a slow commute where sometimes the up-lanes runs a little faster, and sometimes the down-lanes. Balance see-saws....

Velocity of the current has no impact on directionality of the field it produces, it would only effect the strength of the field.

Nik - THis is a physical relationship between current, directional flow and magnetic field production that you cannot discard, change, or mathematically swindle a way out of.

If dynamo theory is correct and the directional flow of molten metal in the earths core generates the earths EM field, the field cannot reverse without a change to the mechanism which produces it. As we all agree on the consequences to the planet as a result of this flow being altered in this way, it simply cannot be the mechanism behind it.

The electrons in Earth's core can carry current easily but not too much heat. The electron's "gas" is degenerated and the classical specific heat (3/2R) can be reached at cca 30.000 K, which is not the case in Earth core (max cca 7.500 K)

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